Add NAWS terminal size negotiation and dynamic dimensions

- Negotiate NAWS (option 31) on connect to receive terminal cols/rows
- Expose term_cols/term_rows on Session (default 80×24)
- Add term_cols/term_rows helpers to TUIRunner::Context
- Make bar() default width dynamic via term_cols

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/bbs/session.rb

@@ -6,7 +6,7 @@ module BBS
   class Session
     include Telnet
 
-    attr_reader :session_id
+    attr_reader :session_id, :term_cols, :term_rows
 
     def initialize(client)
       @client     = client